Determining the best focus position (“best focus”) is an age-old problem for optical apparatus ranging from photographic cameras, to binoculars, to printing apparatus. Determining best focus is particularly troublesome and time consuming in optical apparatus used to manufacture devices. For example, many applications involve processing substrates where the substrate needs to be located within a depth of focus of the optical apparatus that is on the order of microns, and where best focus needs to be determined to up to a fraction of a micron over numerous exposure fields on a substrate.
For example, in photolithography, features on a mask are imaged onto a photosensitive substrate. Many of the mask features are at the resolution limit of the imaging lens (e.g., 0.7λ/NA, where NA is the numerical aperture of the imaging lens). These features will only print over a depth of focus of approximately ±λ/NA2, which is typically only several microns or less. Specific features on the wafer are often used to determine best focus. Sometimes this process involves the time-consuming method of exposing fields on a single substrate at different focus settings, developing the wafer, and then evaluating the sharpness of the printed features.
In laser thermal processing (LTP), high-irradiance radiation (light) is imaged onto individual fields on a highly reflective substrate to be processed. The substrate needs to be within the imaging lens depth of focus of the LTP apparatus, which is generally on the order of tens of microns (e.g., 15 microns).
There are many known methods for determining best focus. Many of the known methods include the steps of forming a bright-field image of a feature at a particular focus position {say, at a point zm along z} and calculating a maximum intensity value Imax and a minimum intensity value Imin. The next step is calculating a contrast function F=(Imax−Imin)/(Imax+Imin). This process is repeated for different focus positions {z1, z2, z3, . . . } surrounding what is thought to be best focus. A maximum value of F versus z is then determined by fitting a curve to the F versus z data points. The value of z that provides the maximum value of F (Fmax) from this curve fit is taken as the best focus position.
Other techniques include comparing the modulation of two objects at different focus positions and determining how to shift focus so that their modulation is equal (e.g., U.S. Pat. No. 4,549,084), a method that compares the image of a second object as formed by a projection optical imaging system at a predetermined plane to a first object (e.g., U.S. Pat. No. 4,952,970), and a method that involves calculating correlation values between images of an object to a pre-stored reference pattern (e.g., U.S. Pat. No. 5,369,430).
However, the need to determine best focus has recently evolved from photolithographic applications, where a predetermined pattern on the substrate is imaged at various focus positions and analyzed, to LTP applications, where a reflective substrate which may not have a pre-determined pattern to be imaged, needs to be placed at best focus quickly and accurately. Unfortunately, highly reflective substrates are difficult to focus using known techniques, because it is difficult to form an image of an object located on a reflective substrate due to the large amount of reflected light.
In addition, best-focus techniques presently applicable to photolithography have several limitations, including being limited to imaging certain patterns having a size at or near the resolution limit of the imaging lens. Further, the methods are not suited for all imaging situations. One such situation is where focus is preferably determined without having to remove the substrate from the imaging field, or where the object (e.g., an alignment mark) is imaged to determine best focus is present on the wafer but the substrate is highly reflective, so that bright-field imaging is not effective. Also, present techniques tend to be time consuming, which limits the ability to process devices or substrates in a timely manner (i.e., limits through-put), and is not generally accurate enough for the most demanding focus situations.